Switched mode power supplies (SMPS) are becoming increasingly common as power supplies for a great variety of applications. For example, SMPS may be used as power supplies for driving LEDs, which may be used to replace incandescent lamps for illumination purposes. However, many other applications for switching power converters exist as practically any electric and electronic device which requires a DC power supply voltage (or current) can be connected to the power grid using SMPS.
LEDs are a type of semiconductor devices that require direct current (DC) for operation. Since the electrical power grid delivers alternating current (AC), a line-powered device must convert the AC to DC in order to power the LEDs. Another increasingly common requirement for line-operated equipment is power factor correction (PFC, also referred to as “power factor control”). Devices which are capable of power factor correction are able to maximize the efficiency of the power grid by making the load “seen” by the power grid appear (approximately) purely resistive thus minimizing the reactive power. The high power factor of resistive loads arises from the unvarying proportionality between the instantaneous voltage and the instantaneous current, i.e., the phase lag between the alternating input voltage and the corresponding alternating input current is approximately zero (and thus the cosine of the phase lag approximately unity).
Usually, PFC circuits form the input stage of a SMPS and are thus coupled between the rectifier (which is present in most SMPS supplied by the AC power grid) and the output stage that is usually a step-down switching power converter (e.g., a buck converter, a resonant converter, or a flyback-converter). The PFC circuit (also referred to as PFC stage of the SMPS) is usually also a switching power converter (e.g., a flyback converter or a boost converter, also referred to as step-up converter). However, the switching operation of the PFC stage is controlled such that the input current follows, on average, the input voltage without (or with a comparably small) phase lag.
PFC circuits may be controlled to perform a quasi-resonant switching. That is, the switching frequency is not defined by a clock. At a given root-mean-square input voltage and at constant load, the switching frequency varies periodically with the double ac line frequency. Furthermore, the frequency range may vary dependent on the electrical load supplied by the SMPS and/or dependent on the input voltage applied to the PFC circuit. Modern SMPS are often designed to cope with a wide range of input voltages and/or with a wide range of electrical loads. As a result the switching frequency of the PFC circuit also varies within a relatively wide frequency range. The switching frequency may assume undesired high values.